1. Field of the Invention
The present invention relates to a method of operating a three-way catalyst of the type which is well known as an exhaust gas purifying device for internal combustion engines and which is mounted in the exhaust system of an internal combustion engine. More particularly, in an internal combustion engine having a three-way catalyst of the type which is mounted in the exhaust system of the engine and containing an element having an oxygen storage capacity (oxygen storage material), a method of operating the three-way catalyst wherein the air-fuel ratio of the air-fuel mixture in the engine intake system (i.e., the mixture supplied to the engine) is set smaller than the stoichiometric ratio, that is, the mixture strength or richness is set on the rich side, and secondary air is supplied intermittently at a certain frequency into the upstream side of the three-way catalyst in the exhaust system so as to alternately vary the air-fuel ratio of the exhaust gases to the rich and lean sides with the stoichiometric ratio as a threshold value.
2. Description of the Prior Art
As a means of simultaneously removing the objectionable components (CO, HC, NO.sub.x) contained in the exhaust gases discharged from an internal combustion engine, the use of a three-way catalyst has been studied extensively in view of the stricter regulations on various exhaust emissions, and many different types of related systems have also been proposed.
As will be seen from the graph shown in FIG. 1 showing the relation between the purification efficiency and the air-fuel ratio, the range within which the threeway catalyst is capable of purifying the three components (CO, HC, NO.sub.x) with high efficiency (i.e., the range of air-fuel ratios in which a high purification percentage of over 80% can be attained and it will hereinafter be referred to as an A/F window) is confined to a very limited range around the stoichiometric air-fuel ratio. As a result, the heretofore proposed exhaust emission control system of the type employing a three-way catalyst are so designed that an air-fuel ratio detector for detecting the stoichiometric ratio is mounted in the exhaust system, and mixture producing means (e.g., the carburetor or electronically controlled fuel injection system) in the intake system is feedback controlled, thus maintaining the air-fuel ratio of mixtures at around the stoichiometric ratio and thereby allowing the three-way catalyst to operate effectively.
With this air-fuel ratio feedback control system, however, it is impossible to always maintain the air-fuel ratio at the stoichiometric one, and the actual air-fuel tends to vary alternately to the rich and lean sides with the stoichiometric ratio as a threshold value. The range or frequency of such variations varies considerably in dependence on the engine operating conditions. As a result, this air-fuel ratio feedback control system is also incapable of ensuring effective utilization of the three-way catalyst.
Recently, catalysts have been developed which are capable of operating with high purification percentages against some variation in the air-fuel ratio. For example, as shown in Japanese Laid-Open Patent Application Publications Tokukai Sho 52-56216, 52-56217, etc., the techniques have been proposed by which platinum (Pt) and rhodium (Rh) in proper proportions are carried on a catalyst bed so as to increase the A/F window of a three-way catalyst. Also disclosed in Japanese Laid-Open Patent Application Publication 52-27087 is the technique by which an oxygen storage material is added to a catalyst so as to similarly ensure an increased A/F window range.
However, these prior art techniques are all designed to improve on the catalysts themselves, and none of these prior art techniques suggest or teach how to operate such three-way catalyst to ensure the most effective operation of the three-way catalyst or how to increase the range of A/F windows for the three-way catalysts.
On the other hand, methods of operating threeway catalysts so as to ensure the effective operation thereof are known in the art as, for example, disclosed in U.S. Pat. No. 4,024,706. In accordance with this U.S. Patent, the air-fuel ratio of the mixture supplied to an engine is varied alternately to the rich and lean sides with the stoichiometric air-fuel ratio as a threshold value to thereby increase the A/F window range of the catalyst. However, the A/F window range that can be obtained with the operating method of this U.S. Patent is as small as 0.18 units in terms of gasoline air-fuel ratio thus making it necessary to employ a feedback control system employing an air-fuel ratio detector, and the method cannot be considered as a satisfactory method of operating a three-way catalyst.
Moreover, the method of the U.S. Patent is disadvantageous in that since the air-fuel ratio of the mixture supplied to the engine (i.e., the mixture in the intake system) is varied alternately to the rich and lean sides, the engine operation tends to become unstable with increase in the variation of the air-fuel ratio and also its effects increase with decrease in the frequency of the variations. Thus, the three-way catalyst operating method disclosed in the U.S. Patent has a deteriorating effect on the engine operation.